Home Search Collections Journals About Contact us My IOPscience Formation of TiO2 nanostructure by plasma electrolytic oxidation for Cr(VI) reduction This content has been downloaded from IOPscience Please scroll down to see the full text 2017 J Phys.: Conf Ser 786 012046 (http://iopscience.iop.org/1742-6596/786/1/012046) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 46.161.58.119 This content was downloaded on 13/02/2017 at 07:51 Please note that terms and conditions apply You may also be interested in: The effect of plasma electrolytic oxidation on the mean stress sensitivity of the fatigue life of the 6082 aluminum alloy L Winter, R Morgenstern, K Hockauf et al Effects of CH3OH Addition on Plasma Electrolytic Oxidation of AZ31 Magnesium Alloys He Yongyi, Chen Li, Yan Zongcheng et al Microstructure and Corrosion Performance of Oxide Coatings on Aluminium byPlasma Electrolytic Oxidation in Silicate and Phosphate Electrolytes Lv Guo-Hua, Gu Wei-Chao, Chen Huan et al Application of Traditional and Nanostructure Materials for Medical Electron Beams Collimation: Numerical Simulation I A Miloichikova, S G Stuchebrov, G K Zhaksybayeva et al Studies on Nanostructure Aluminium Thin Film Coatings Deposited using DC magnetron Sputtering Process Muralidhar Singh M, Vijaya G, Krupashankara MS et al Flame-made ultra-porous TiO2 layers for perovskite solar cells Yahuitl Osorio Mayon, The Duong, Noushin Nasiri et al Patterning of titanium oxide nanostructures by electron-beam lithography combined with plasma etching I Hotovy, I Kostic, P Nemec et al CCEQ IOP Conf Series: Journal of Physics: Conf Series 786 (2017) 012046 IOP Publishing doi:10.1088/1742-6596/786/1/012046 International Conference on Recent Trends in Physics 2016 (ICRTP2016) IOP Publishing Journal of Physics: Conference Series 755 (2016) 011001 doi:10.1088/1742-6596/755/1/011001 Formation of TiO2 nanostructure by plasma electrolytic oxidation for Cr(VI) reduction D A Torres1, F Gordillo-Delgado1 and J Plazas-Salda1 Universidad del Quindío, Armenia, Colombia E-mail: fgordillo@uniquindio.edu.co Abstract Plasma electrolytic oxidation (PEO) is an environmentally friendly technique that allows the growth of ceramic coatings without organic solvents and non-toxic residues This method was applied to ASME SB-265 titanium (Ti) plates (2×2×0.1cm) using voltage pulses from a switching power supply (340V) for 10 minutes at frequency of 1000Hz changing duty cycle at 10, 60 and 90% and the electrolytes were Na3PO4 and NaOH The treated sheets surfaces were analysed by X-ray diffraction and scanning electron microscopy According to the diffractograms, the duty cycle increase produces amorphous TiO2 coating on Ti sheets and the thickness increases After sintering at 900°C during hour, the 10% duty cycle generated a combination of anatase and rutile phases at the sample surface with weight percentages of 13.3 and 86.6% and particle sizes of 32.461±0.009nm and 141.14±0.03 nm, respectively With this sample, the total reduction of hexavalent chromium was reached at 50 minutes for 1ppm solution This photocatalytic activity was measured following the colorimetric method ASTM3500-Cr B Introduction The surface modification of metal substrates and alloys using plasma electrolytic oxidation (PEO) for the formation of ceramic coatings with high hardness and large surface area is an environmentally friendly technique, simple for the porous material production and low-cost In PEO, microdischarges are produced by high potential difference between the metal sheet and another electrode and modifying the frequency or duty cycle of the voltage pulses, which are important parameters for determining the growth characteristics and morphology of the oxide layer [1] Titanium dioxide (TiO2) is a semiconductor widely studied due to its large variety of applications such as environmental purification, sewage treatment, metal reduction, biomedical implants and others TiO2 has three crystallographic phases: rutile, anatase and brookite, which play an important role according to applications [2,3] The TiO2 photocatalytic activity is a solution to attack environmental problems related to contamination by high metals as hexavalent chromium (Cr(VI)); especially, it is one of the most notorious in tanning industry, mining activity and pigment manufacture causing health problems by ingestion, inhalation or absorption through the skin [4,5] Materials and methods 2.1 Preparation of samples and TiO2 coatings growth Titanium sheets, 99% purity grade (ASME SB-265), with dimensions 20×20×0.1mm were used in this work Before to PEO, the plates were polished with 1000 and 1200 grit SiC sandpapers successively and subsequently cleaned by ultrasonic bath (BRANSON 1510, Colombia) in distilled water for 30 Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI Published under licence by IOP Publishing Ltd CCEQ IOP Conf Series: Journal of Physics: Conf Series 786 (2017) 012046 IOP Publishing doi:10.1088/1742-6596/786/1/012046 minutes A DC pulse power supply designed to deliver a maximum peak voltage of 340V between the electrodes, with variable frequency of 1000Hz and duty cycle of 10, 60 or 90%, was employed in the PEO system A water cooled stainless steel chamber, containing an aqueous electrolyte g/L Na3PO4 and 0.4g/LNaOH, was used as electrode and the Ti sheet was the other electrode Subsequently, the sintering of the samples was performed at 500, 700 and 900°C for one hour [1] 2.2 Characterization of TiO2 The coatings’ thickness was measured using a micrometer (Mitutoyo 543-460B Digital, Colombia) with an accuracy of ±0.1μm The thickness was measured at 20 different locations on the sample surface The phase composition of the coatings was studied by glancing angle XRD measurements in parallel beam geometry using X-ray diffractometer (XBruker D8- Advance, Colombia) The Pictures of the samples’ surface were taken using a scanning electron microscopy (SEM, JEOL JSM-6390LV, CICATA, México) [6] 2.3 Photocatalytic activity The analysis of the photocatalytic activity reduction of Cr(VI) to Cr(III) was done following the colorimetric method (ASTM-3500 B-Cr) with 20 mL aqueous Cr(VI) solution with a concentration of 1ppm The light from 125W UV-lamp, which has main emission at 253.7nm, was used to radiate the sample in contact with the solution into a beaker; TiO2/Ti catalyst was placed in the middle of this beaker and the process was completed at two hours The TiO2 coating absorbs UV light so the electrons are excited from valence band to conduction band; this electron-hole pair generates oxidization and radicals OH- [7,8] as seen in the Equations (1) and (2): O" + 4H" O + 4e' )*+, /./ 4OH ' ' ' CrO"' + 4H" O + 3e → Cr(OH)7 + 5OH (1) (2) A UV–Vis spectrophotometer (Thermo Scientific Evolution 201/220 UV-Vis system, Colombia) was used to measure the absorbance of the treated solutions under the maximum absorbance wavelength (540nm) In the range of low concentration, the reduction rate in the solution of Cr (VI) to Cr (III) was calculated through the Equation (3): Removal rate % = C0 - C 9: x 100 % (3) Where C is the concentration of Cr(VI) at a certain interval of time (mg/L) and C0 is the initial concentration of Cr(VI) [5] Results and discussion 3.1 Characterization of TiO2 coatings During PEO, white sparks appear when critical voltage is achieved; later the tension stabilizes and orange sparks appear as in Figure [9] The coating thickness presents a progressive increase with the duty cycle 10, 60 and 90% of 0.32%, 5.4% and 9.49%, respectively at 1000Hz [10] We assumed that this thickness increase occurs on ton and solidifies during toff; this suggests that the heat is absorbed by the coating and the micro-discharge colour changes The coatings by PEO were examined using SEM, revealing uniform microstructures by voltage pulse frequency 1000Hz and duty cycle of 10%, as shown in Figure When this frequency is applied the pores density is large enough; because of the high density of the sparks [11] The pores diameter was approximately between 356 and 1200nm The CCEQ IOP Conf Series: Journal of Physics: Conf Series 786 (2017) 012046 IOP Publishing doi:10.1088/1742-6596/786/1/012046 weight and atomic chemical composition were determined by EDS [12] giving 48.66% of oxygen, 43.06% of titanium and 8.28% of phosphorus The phosphorus presence in the films shows that electrolyte enters in discharge channels during the growth process; although SEM analysis shows a significant amount of phosphorus, it was not found by XRD, because it is in amorphous shape or the crystalline component may be below diffractometer detection limit [11] Figure Formation of orange sparks during the TiO2 superficial coating by PEO on the Ti plate (a) (b) Figure Surface morphology by SEM of TiO2 on Ti sheet at 1000Hz, duty cycle of 10% and 500°C: (a) low magnification x500 and (b) high magnification x10000 The XRD patterns of titanium substrate and coatings (10, 60 and 90% duty cycles) are shown in Figure 3, where peaks of titanium and amorphous TiO2 can be seen (2θ between 20° and 30°) Sample diffractograms after thermal treatment at 500, 700 and 900°C can be seen in Figure The average particle size was determined by the Scherrer Equation (4), involving the Bragg angle (2θ) and the Full Width at Half Maximum (FWHM); the values were listed in Table [13] T= ;.= > ?@AB CDE F (4) IOP Publishing doi:10.1088/1742-6596/786/1/012046 CCEQ IOP Conf Series: Journal of Physics: Conf Series 786 (2017) 012046 From Figure 5, anatase and rutile weight fractions during sintering of TiO2 coatings were calculated using the Spurr and Myers’s Equations, (5) and (6) [14]: I (5) XH= LM IJI."K LN X O =1- X H (6) Where XA and XR are the phase’s percentages and IA and IR are the maximum-intensity of the XRD peaks of anatase (101) and rutile (110) phases The results are shown in Table 1000 Hz (10%) 700 ºC 1000 Hz (90%) 2000 50 1000 0 50 1000 Hz (10% ) 500 ºC 100 XRD Intensity (a.u) XRD Intensity (a.u) 1000 Hz (90% ) 1000 Hz (10%) 100 1000 Hz (10%) 100 900 900 Titanium Titanium 600 600 300 300 20 30 40 50 20 60 30 2θ (degree) 40 50 60 Angle (2θ) Figure Diffractograms of TiO2 coatings; PEO with duty cycle: 10, 60 and 90% Figure Diffractograms of TiO2 coatings; PEO with 1000 Hz and 10% duty cycle and different sintering temperature Table Particle size; phase percentage and reduction of Cr (VI) using Ti sheets modified by PEO with 1000 Hz and 10 % of duty cycle Sintering Anatase average size Rutile average size Anatase Temperature (ºC) (nm) (nm) (%) 500 700 900 16.567±0.004 24,40±0.01 32.461±0.009 73.819±0.069 141.143±0.029 100 75.2 13.3 Rutile (%) Percentage reduction of Cr (%) 24.6 86.6 99.488±0.004 98.848±0.004 26.442±0.004 IOP Publishing doi:10.1088/1742-6596/786/1/012046 CCEQ IOP Conf Series: Journal of Physics: Conf Series 786 (2017) 012046 0,8 R(110) 8000 Absorbance (a.u.) R(220) R(211) Ti A(200) R(210) R(101) Ti 2000 R(200) Ti R(111) 4000 A (101) XRD Intensity (a.u.) 0,6 6000 0,4 0,2 0,0 0,0 20 25 30 35 40 2θ (degree) 45 50 55 0,2 0,4 0,6 0,8 1,0 Cr(VI) concentration (ppm) 60 Figure Diffractogram of TiO2 films sintered at 900 °C and formation of crystalline anatase and rutile phase Figure Calibration curve for quantification of Cr (VI) reduction 3.2 Photocatalytic activity For the evaluation of the photocatalytic activity of TiO2 coatings, a 1ppm Cr(VI) calibration curve was done as shown in Figure 6, using Standard Methods 3500-Cr Absorption capacity of coated plates was analysed locating it into the same solution in darkness The Cr(VI) reduction performance with UV irradiation at hours is reported in Table 1; for 900°C sintering, a deterioration of the coating was found and this sample had the lowest performance (approximately 26%); while at 500°C sintering, the sample had the highest performance (approximately 99%) However, this value was achieved after 50 and 90 minutes for 1ppm and 10ppm Cr(VI) respectively, according to Figure that shows reduction percentage in function of time 100 ppm 10 ppm Dark Cr (VI) reduction (%) 80 60 40 20 0 20 40 60 80 100 120 Time (min) Figure Percent reduction of ppm (squared dots) and 10 ppm (circle dots) Cr(VI) and the effect by solution adsorption in the darkness (triangle dots) CCEQ IOP Conf Series: Journal of Physics: Conf Series 786 (2017) 012046 IOP Publishing doi:10.1088/1742-6596/786/1/012046 Conclusion TiO2 ceramic coatings on titanium sheets were accomplished by PEO technique using voltage pulses frequency 1000Hz and changing the duty cycle These porous films were sintering for growing TiO2 nanostructures of a mixed phase with particle sizes between 16 and 32nm for anatase and between 74 and 141nm for rutile phase; the SEM pictures showed a topography of microcavities; the EDS analysis shows small phosphorus amounts, which were not detected by XRD, but it could be present in the sample as an impurity The equilibrium time of maximum adsorption (7%) was 30 minutes and a reduction almost complete was achieved after 50 minutes of radiation with the TiO2 coating made with 10% of duty cycle Acknowledgments The authors thank to the University of Quindío for the financial support through the project 805 and the research group’s incentive program Special thanks to Luis Salazar from the Chemistry department for the technical assistance with Uv-Vis absorbance measurements, to Dr Liliana Tirado and Angélica Marcela Castillo from the Interdisciplinary Institute of Sciences for the XRD measurements and to E Marín and A Bedoya from CICATA-IPN, México for the SEM pictures References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] Arunnellaiappan T & Kishore N 2015 Influence of frequency and duty cycle on microstructure of plasma electrolytic oxidized AA7075 and the correlation to its corrosion behavior Surface and Coatings Technology 280 136-147 Franz S & Perego D 2016 Photoactive TiO2 coatings obtained by Plasma Electrolytic Oxidation in refrigerated electrolytes Applied Surface Science 385 498-505 Lekphet W & Tsai-Chyuan Ke 2016 Morphology control studies of TiO2 microstructures via surfactantassisted hydrothermal process for dye-sensitized solar cell applications Applied Surface Science 382 1526 Zhou Q & Jing Li Z 2015 Applications of TiO2 nanotube arrays in environmental and energy fields: A review Microporous and Mesoporous Materials 202 22-35 Zhongping Yao, Fangzhou Jia & Yanli J 2010 Photocatalytic reduction of potassiumchromate by Zndoped TiO2/Ti film catalyst Applied Surface Science 256 1793-1797 Mohammad K & Hossein S 2014 Effect of heat treatment temperature on the performance of nano-TiO2 coating in protecting 316L stainless steel against corrosion under UV illumination and dark conditions Surface and Coatings Technology 258 861-870 Qing S & Xiaolong H 2015 Influence of calcination temperature on the structural adsorption and photocatalytic properties of TiO2 nanoparticles supported on natural zeolite Powder Technology 274 8897 Yang J & Lee S 2012 Photocatalytic removal of CrVI with illuminated TiO2 Desalination and Water Treatment 46 375-380 Venkateswarlu K & Rameshbabu N 2012 Role of electrolyte additives on in-vitro electrochemical behavior of micro arc oxidized titania films on Cp Ti Applied Surface Science 258 6853-6863 Vahid D & Ben L 2013 Effect of duty cycle and applied current frequency on plasma electrolytic oxidation Surface & Coatings Technology 226 100-107 Gowtham S, Arunnellaiappan T & Rameshbabu N 2016 An investigation on pulsed DC plasma electrolytic oxidation of cp-Ti and its corrosion behaviour in simulated body fluid Surface and Coatings Technology 301 63-73 Nenad T & Stevan S 2016 Characterization and photocatalytic properties of tungsten doped TiO2 coatings on aluminum obtained by plasma electrolytic oxidation Surface and Coatings Technology 305 192-199 Allen, B, Kenneth, O & Thomas, R 2009 On the estimation of average crystallite size of zeolites from the Scherrer equation: A critical evaluation of its application to zeolites with one-dimensional pore systems Microporous and Mesoporous Materials 117 75-90 Roach M & Williamson R 2016 Tuning anatase and rutile phase ratios and nanoscale surface features by anodization processing onto titanium substrate surfaces Materials Science and Engineering: C 58 213 ... Series 755 (2016) 011001 doi:10.1088/1742-6596/755/1/011001 Formation of TiO2 nanostructure by plasma electrolytic oxidation for Cr( VI) reduction D A Torres1, F Gordillo-Delgado1 and J Plazas-Salda1... 1,0 Cr( VI) concentration (ppm) 60 Figure Diffractogram of TiO2 films sintered at 900 °C and formation of crystalline anatase and rutile phase Figure Calibration curve for quantification of Cr (VI) ... surface modification of metal substrates and alloys using plasma electrolytic oxidation (PEO) for the formation of ceramic coatings with high hardness and large surface area is an environmentally friendly